sunrpc: fix prog selection loop in svc_process_common

If the rq_prog is not in the list of programs, then we use the last
program in the list and we don't get the expected rpc_prog_unavail error
as the subsequent tests on 'progp' being NULL are ineffective.

We should only assign progp when we find the right program, and we
should initialize it to NULL

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: 86ab08be ("SUNRPC: replace program list with program array")
Signed-off-by: NeilBrown <neilb@suse.de>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>